Deep isolating trenches formed in a substrate are intended to isolate the different elements of the circuit from each other, and to minimize parasitic components that exist between the structures. These deep isolating trenches are known as DTI (Deep Trench Isolation). Deep means that the depth of the trenches is greater than their width, and is much greater than the depth of the buried layers in the substrate. These deep trenches are used to separate the N+ and P+ buried layers, and thus reduce the perimeter component of the collector/substrate capacitance. These capacitances contribute to the calculation of circuit noise and to the oscillation frequency of bipole transistors.
Deep isolating trenches surround bipole transistors and are full of electrically insulating material. Silicon is by far the most frequently used material in the microelectronics industry, and the material used in silicon circuits is usually silicon oxide.
There are various methods of forming these deep isolating trenches. They may be formed at the end of a process after the transistors have been made, or at the beginning of the process before the transistors are made.
When the trenches are formed at the end of the process, they have a large perimeter due to the fact that there is obviously no need to cut the elements of the transistor. The surface component of the collector/substrate capacitance is the same with or without a deep trench. The gain on the perimeter component is due to the fact that the value of the capacitance of the trench per unit length is less than the junction capacitance when there is no trench. The total gain on the collector/substrate capacitance is limited with this type of trench.
If trenches are formed at the beginning of the process, their perimeter can be limited since, for example, the contact point of the base can be stacked above the trench. The collector/substrate perimeter capacitance is lower than in the previous configuration. But once the trenches have been filled with electrically insulating material, a number of thermal cycles will be applied to the substrate to produce transistors or other components. These cycles will generate high mechanical stresses between the substrate and the filling material. These stresses are generated by the differences in the coefficients of thermal expansion between the silicon and the trench filling material. The difference in coefficients of thermal expansion between silicon and the silicon dioxide usually used to fill these trenches is on the order of a factor of 10.
Beyond a threshold stress, dislocations can be generated in the substrate material. When the dislocations intersect the junctions, junction leakage currents are very high and the circuit is unusable. This disadvantage is very limited if the trenches are formed at the end of the process, since in this case, the circuit is no longer subjected to high temperature cycles after the trenches have been filled.
A composite filling with two different materials to limit the development of these dislocations has been proposed if the trenches are formed at the beginning of the process. For example, the sides and bottom can be covered with a layer of oxide and the entire interior can be filled with polycrystalline silicon. But the value of the capacitance per unit length of a composite filling is greater than the value of a filling with oxide. This is because the relative permittivity of polycrystalline silicon is three or four times greater than the relative permittivity of the silicon dioxide.